1. Field of the Invention
The present invention relates to a constant voltage output circuit, and more particularly to a constant voltage output circuit that is supplied with electric power from a plurality of power sources.
2. Description of Related Art
Conventionally, constant voltage output circuits are provided with an overcurrent protection circuit and a short-circuiting protection circuit so that, even if their output power transistor happens to output excessive electric power above its rated operating level, the load connected to them is not destroyed. Examples of constant voltage output circuits provided with a protection circuit are proposed, for example, in JP-A-2005-293067 (hereinafter Patent Document 1), pp. 4-5 and FIG. 1 and in JP-A-2001-216037 (hereinafter Patent Document 2), pp. 5-7 and FIG. 1. The power regulator of Patent Document 2 is provided with two input power sources so that its output power transistor and control circuit are supplied with electric power from different input power sources.
FIG. 8 is a block diagram of a conventional constant voltage output circuit provided with an overcurrent protection circuit and a short-circuiting protection circuit. The constant voltage output circuit 10 shown in FIG. 8 is provided with an input power source VCC2, an input power source VCC1, a control circuit 20 including an operational amplifier, an output power transistor 12, an output terminal Vo, an overcurrent protection circuit 15 and a short-circuiting protection circuit 16. The constant voltage output circuit 10 supplies a voltage to a load (unillustrated) connected to the output terminal Vo.
In the constant voltage output circuit 10, the control circuit 20 is supplied with electric power from the input power source VCC2. The output power transistor 12 is an NPN-type bipolar transistor. The output power transistor 12 receives at its collector the output voltage of the input power source VCC1, and is at its emitter grounded via a serial circuit composed of voltage division resistors 13 and 14. The emitter of the output power transistor 12 is also connected to the output terminal Vo.
The node between the voltage division resistors 13 and 14 is connected to the inverting input terminal (−) of the operational amplifier of the control circuit 20. The operational amplifier of the control circuit 20 receives at its non-inverting input terminal (+) a reference voltage Vref generated by a power source 17. The output of the operational amplifier, i.e. the output of the control circuit 20, is fed to the base of the output power transistor 12.
The overcurrent protection circuit 15 is connected between the input power source VCC1 and the control circuit 20, and the short-circuiting protection circuit 16 is connected between the emitter of the output power transistor 12 and the control circuit 20. The overcurrent protection circuit 15 and the short-circuiting protection circuit 16 are both supplied with electric power from the input power source VCC1. The overcurrent protection circuit 15 monitors the current flowing through the output power transistor 12, and operates so that the current does not exceed a predetermined level. Even if the output terminal Vo happens to be short-circuited to ground and accordingly the potential at the inverting input terminal of the operational amplifier drops, the short-circuiting protection circuit 16 prevents the output power transistor 12 from being driven at an excessively high operating level. Without these protection circuits, the output power transistor 12 may dissipate excessive electric power and break down.
Inconveniently, however, a conventional protection circuit operates only when a voltage higher than a predetermined level is supplied. Thus, in a constant voltage output circuit that is supplied with electric power from a single input power source, when the input power source is turned on from a state in which no voltage is present there, that is, when the input power source is turned on from a state in which it is completely off, a protection circuit does not operate until the supplied voltage becomes equal to or higher than a predetermined level at or above which individual circuit can operate. That is, in a case where a single input power source is used, unless a voltage equal to or higher than a predetermined level is present, a protection circuit does not operate. Even then, the circuit for driving an output transistor does not operate either; thus, the output transistor is not driven at a higher-than-rated operating level. Consequently, no problem results from the failure of the protection circuit to operate.
On the other hand, in a case where there are two or more input power sources, for example in a case where, as in the constant voltage output circuit shown in FIG. 8, there are one input power source VCC2 for supplying electric power to the control circuit 20 and driving an overcurrent protection circuit 15 and a short-circuiting protection circuit 16 and another input power source VCC1 for supplying a voltage to the collector of an output power transistor 12, if, from a state in which both input power sources are off, the input power source VCC1 is turned on first and then the input power source VCC2 is turned on, there occurs a period in which, while the overcurrent protection circuit 15 and the short-circuiting protection circuit 16 are not operating, the output power transistor 12 is driven. If the input power sources VCC2 and VCC1 start up in this order when, for example, the output terminal Vo happens to be short-circuited, the output power transistor 12 may break down.